Recently, cellular phones having touch-panel input devices have been spread all over the world. Touch panels are combinations of display devices such as LCD (Liquid Crystal Display) and input devices such as touch pads. Touch-panel cellular phones guarantee intuitive operations for users to touch the touch panels with fingers without complex operations needed; hence, they are favorable for users in comparison with conventional cellular phones needing operations with buttons and switches.
Recent cellular phones incorporate touch panels covering the entire surfaces of cellular phones with large screen sizes. Since the entire surface screens of touch panels function as input devices, users holding cellular phones while touching the touch panels with fingers or hands may cause error operations to be accepted by cellular phones. To prevent error operations, cellular phones can be designed not to accept an operation at a second touch position when a user concurrently touches two positions on a touch panel. This type of cellular phones may incur another possibility in that, when a touch panel is touched by a user's finger or a user's hand holding a cellular phone, the cellular phone cannot recognize a user's operation of touching the touch panel of the cellular phone with the other finger or the other hand. Additionally, when a user holds a cellular phone without touching a touch panel with a finger or a hand, a cellular phone may be hardly maintained at a desired position in space and become unstable in positioning, thus incurring the possibility that the user may drop the cellular phone on the floor by mistake.
Various technologies have been developed to solve the above problems. For example, Patent Literature 1 discloses a touch-panel information processing device, which executes a function assigned to a touch area on a touch panel when an amount of a touch time a user touches the touch panel with a finger or a hand does not reach a predetermined time but which does not execute the function assigned to the touch area on the touch panel when an amount of a touch time the user touches the touch panel reaches the predetermined time. This is based on a prediction that, when a user holding a cellular phone touches a touch panel with a finger or a hand, the user may be touching the touch panel with a finger for a longer period of time than a touch time assigned to a normal input operation. For this reason, it is possible to reduce the frequency of a user's error operations on a touch panel by use of the method disclosed in Patent Literature 1.
Patent Literature 2 discloses a touch-panel cellular phone, which makes it easy for a user to operate a touch panel with a user's finger or a user's hand holding a cellular phone because the cellular phone is equipped with a plurality of sensors used for detecting a holding position of the cellular phone in the longitudinal direction so as to shift a key display area and display it on the touch panel in correspondence with the holding position.